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If these are generic example addresses, they should be changed to use the 233.252.0.x range defined in RFC 5771 Miscellaneous warnings: ---------------------------------------------------------------------------- == The copyright year in the IETF Trust and authors Copyright Line does not match the current year == The document seems to lack the recommended RFC 2119 boilerplate, even if it appears to use RFC 2119 keywords. (The document does seem to have the reference to RFC 2119 which the ID-Checklist requires). -- The document date (July 12, 2021) is 1018 days in the past. Is this intentional? 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Haberman, Ed. 3 Internet-Draft JHU APL 4 Obsoletes: 3376 (if approved) July 12, 2021 5 Intended status: Standards Track 6 Expires: January 13, 2022 8 Internet Group Management Protocol, Version 3 9 draft-mcast-pim-3376bis-00 11 Abstract 13 This document specifies a revised Version 3 of the Internet Group 14 Management Protocol, IGMPv3. IGMP is the protocol used by IPv4 15 systems to report their IP multicast group memberships to neighboring 16 multicast routers. Version 3 of IGMP adds support for source 17 filtering, that is, the ability for a system to report interest in 18 receiving packets only from specific source addresses, or from all 19 but specific source addresses, sent to a particular multicast 20 address. That information may be used by multicast routing protocols 21 to avoid delivering multicast packets from specific sources to 22 networks where there are no interested receivers. 24 This document obsoletes RFC 3376. 26 Status of This Memo 28 This Internet-Draft is submitted in full conformance with the 29 provisions of BCP 78 and BCP 79. 31 Internet-Drafts are working documents of the Internet Engineering 32 Task Force (IETF). Note that other groups may also distribute 33 working documents as Internet-Drafts. The list of current Internet- 34 Drafts is at https://datatracker.ietf.org/drafts/current/. 36 Internet-Drafts are draft documents valid for a maximum of six months 37 and may be updated, replaced, or obsoleted by other documents at any 38 time. It is inappropriate to use Internet-Drafts as reference 39 material or to cite them other than as "work in progress." 41 This Internet-Draft will expire on January 13, 2022. 43 Copyright Notice 45 Copyright (c) 2021 IETF Trust and the persons identified as the 46 document authors. All rights reserved. 48 This document is subject to BCP 78 and the IETF Trust's Legal 49 Provisions Relating to IETF Documents 50 (https://trustee.ietf.org/license-info) in effect on the date of 51 publication of this document. Please review these documents 52 carefully, as they describe your rights and restrictions with respect 53 to this document. Code Components extracted from this document must 54 include Simplified BSD License text as described in Section 4.e of 55 the Trust Legal Provisions and are provided without warranty as 56 described in the Simplified BSD License. 58 Table of Contents 60 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4 61 2. The Service Interface for Requesting IP Multicast Reception . 5 62 3. Multicast Reception State Maintained by Systems . . . . . . . 6 63 3.1. Socket State . . . . . . . . . . . . . . . . . . . . . . 6 64 3.2. Interface State . . . . . . . . . . . . . . . . . . . . . 7 65 4. Message Formats . . . . . . . . . . . . . . . . . . . . . . . 9 66 4.1. Membership Query Message . . . . . . . . . . . . . . . . 10 67 4.1.1. Max Resp Code . . . . . . . . . . . . . . . . . . . . 11 68 4.1.2. Checksum . . . . . . . . . . . . . . . . . . . . . . 12 69 4.1.3. Group Address . . . . . . . . . . . . . . . . . . . . 12 70 4.1.4. Resv (Reserved) . . . . . . . . . . . . . . . . . . . 12 71 4.1.5. S Flag (Suppress Router-Side Processing) . . . . . . 12 72 4.1.6. QRV (Querier's Robustness Variable) . . . . . . . . . 12 73 4.1.7. QQIC (Querier's Query Interval Code) . . . . . . . . 12 74 4.1.8. Number of Sources (N) . . . . . . . . . . . . . . . . 13 75 4.1.9. Source Address [i] . . . . . . . . . . . . . . . . . 13 76 4.1.10. Additional Data . . . . . . . . . . . . . . . . . . . 13 77 4.1.11. Query Variants . . . . . . . . . . . . . . . . . . . 14 78 4.1.12. IP Destination Addresses for Queries . . . . . . . . 14 79 4.2. Version 3 Membership Report Message . . . . . . . . . . . 14 80 4.2.1. Reserved . . . . . . . . . . . . . . . . . . . . . . 16 81 4.2.2. Checksum . . . . . . . . . . . . . . . . . . . . . . 16 82 4.2.3. Number of Group Records (M) . . . . . . . . . . . . . 16 83 4.2.4. Group Record . . . . . . . . . . . . . . . . . . . . 16 84 4.2.5. Record Type . . . . . . . . . . . . . . . . . . . . . 17 85 4.2.6. Aux Data Len . . . . . . . . . . . . . . . . . . . . 17 86 4.2.7. Number of Sources (N) . . . . . . . . . . . . . . . . 17 87 4.2.8. Multicast Address . . . . . . . . . . . . . . . . . . 17 88 4.2.9. Source Address [i] . . . . . . . . . . . . . . . . . 17 89 4.2.10. Auxiliary Data . . . . . . . . . . . . . . . . . . . 17 90 4.2.11. Additional Data . . . . . . . . . . . . . . . . . . . 17 91 4.2.12. Group Record Types . . . . . . . . . . . . . . . . . 18 92 4.2.13. IP Source Addresses for Reports . . . . . . . . . . . 19 93 4.2.14. IP Destination Addresses for Reports . . . . . . . . 19 94 4.2.15. Notation for Group Records . . . . . . . . . . . . . 20 95 4.2.16. Membership Report Size . . . . . . . . . . . . . . . 20 97 5. Description of the Protocol for Group Members . . . . . . . . 21 98 5.1. Action on Change of Interface State . . . . . . . . . . . 22 99 5.2. Action on Reception of a Query . . . . . . . . . . . . . 24 100 6. Description of the Protocol for Multicast Routers . . . . . . 26 101 6.1. Conditions for IGMP Queries . . . . . . . . . . . . . . . 27 102 6.2. IGMP State Maintained by Multicast Routers . . . . . . . 28 103 6.2.1. Definition of Router Filter-Mode . . . . . . . . . . 28 104 6.2.2. Definition of Group Timers . . . . . . . . . . . . . 29 105 6.2.3. Definition of Source Timers . . . . . . . . . . . . . 30 106 6.3. IGMPv3 Source-Specific Forwarding Rules . . . . . . . . . 31 107 6.4. Action on Reception of Reports . . . . . . . . . . . . . 32 108 6.4.1. Reception of Current-State Records . . . . . . . . . 32 109 6.4.2. Reception of Filter-Mode-Change and Source-List- 110 Change Records . . . . . . . . . . . . . . . . . . . 33 111 6.5. Switching Router Filter-Modes . . . . . . . . . . . . . . 35 112 6.6. Action on Reception of Queries . . . . . . . . . . . . . 36 113 6.6.1. Timer Updates . . . . . . . . . . . . . . . . . . . . 36 114 6.6.2. Querier Election . . . . . . . . . . . . . . . . . . 36 115 6.6.3. Building and Sending Specific Queries . . . . . . . . 36 116 7. Interoperation With Older Versions of IGMP . . . . . . . . . 37 117 7.1. Query Version Distinctions . . . . . . . . . . . . . . . 38 118 7.2. Group Member Behavior . . . . . . . . . . . . . . . . . . 38 119 7.2.1. In the Presence of Older Version Queriers . . . . . . 38 120 7.2.2. In the Presence of Older Version Group Members . . . 39 121 7.3. Multicast Router Behavior . . . . . . . . . . . . . . . . 40 122 7.3.1. In the Presence of Older Version Queriers . . . . . . 40 123 7.3.2. In the Presence of Older Version Group Members . . . 40 124 8. List of Timers, Counters and Their Default Values . . . . . . 42 125 8.1. Robustness Variable . . . . . . . . . . . . . . . . . . . 42 126 8.2. Query Interval . . . . . . . . . . . . . . . . . . . . . 42 127 8.3. Query Response Interval . . . . . . . . . . . . . . . . . 43 128 8.4. Group Membership Interval . . . . . . . . . . . . . . . . 43 129 8.5. Other Querier Present Interval . . . . . . . . . . . . . 43 130 8.6. Startup Query Interval . . . . . . . . . . . . . . . . . 43 131 8.7. Startup Query Count . . . . . . . . . . . . . . . . . . . 43 132 8.8. Last Member Query Interval . . . . . . . . . . . . . . . 43 133 8.9. Last Member Query Count . . . . . . . . . . . . . . . . . 44 134 8.10. Last Member Query Time . . . . . . . . . . . . . . . . . 44 135 8.11. Unsolicited Report Interval . . . . . . . . . . . . . . . 44 136 8.12. Older Version Querier Present Interval . . . . . . . . . 44 137 8.13. Older Host Present Interval . . . . . . . . . . . . . . . 44 138 8.14. Configuring Timers . . . . . . . . . . . . . . . . . . . 45 139 8.14.1. Robustness Variable . . . . . . . . . . . . . . . . 45 140 8.14.2. Query Interval . . . . . . . . . . . . . . . . . . . 45 141 8.14.3. Max Response Time . . . . . . . . . . . . . . . . . 45 142 9. Security Considerations . . . . . . . . . . . . . . . . . . . 46 143 9.1. Query Message . . . . . . . . . . . . . . . . . . . . . . 46 144 9.2. Current-State Report messages . . . . . . . . . . . . . . 47 145 9.3. State-Change Report Messages . . . . . . . . . . . . . . 48 146 9.4. 9.4. IPSEC Usage . . . . . . . . . . . . . . . . . . . . 48 147 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 49 148 11. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 49 149 12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 49 150 13. References . . . . . . . . . . . . . . . . . . . . . . . . . 49 151 13.1. Normative References . . . . . . . . . . . . . . . . . . 49 152 13.2. Informative References . . . . . . . . . . . . . . . . . 50 153 Appendix A. Design Rationale . . . . . . . . . . . . . . . . . . 50 154 A.1. The Need for State-Change Messages . . . . . . . . . . . 50 155 A.2. Host Suppression . . . . . . . . . . . . . . . . . . . . 51 156 A.3. Switching Router Filter Modes from EXCLUDE to INCLUDE . . 51 157 Appendix B. Summary of Changes from IGMPv2 . . . . . . . . . . . 52 158 Appendix C. Summary of Changes from RFC 3376 . . . . . . . . . . 53 159 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 53 161 1. Introduction 163 The Internet Group Management Protocol (IGMP) is used by IPv4 systems 164 (hosts and routers) to report their IP multicast group memberships to 165 any neighboring multicast routers. Note that an IP multicast router 166 may itself be a member of one or more multicast groups, in which case 167 it performs both the multicast router part of the protocol (to 168 collect the membership information needed by its multicast routing 169 protocol) and the group member part of the protocol (to inform itself 170 and other, neighboring multicast routers of its memberships). 172 IGMP is also used for other IP multicast management functions, using 173 message types other than those used for group membership reporting. 174 This document specifies only the group membership reporting functions 175 and messages. 177 This document specifies Version 3 of IGMP. Version 1, specified in 178 [RFC1112], was the first widely-deployed version and the first 179 version to become an Internet Standard. Version 2, specified in 180 [RFC2236], added support for low leave latency, that is, a reduction 181 in the time it takes for a multicast router to learn that there are 182 no longer any members of a particular group present on an attached 183 network. Version 3 adds support for source filtering, that is, the 184 ability for a system to report interest in receiving packets only 185 from specific source addresses, as required to support Source- 186 Specific Multicast [RFC3569], or from all but specific source 187 addresses, sent to a particular multicast address. Version 3 is 188 designed to be interoperable with Versions 1 and 2. 190 This document obsoletes [RFC3376]. 192 The capitalized key words "MUST", "MUST NOT", "REQUIRED", "SHALL", 193 "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and 194 "OPTIONAL" in this document are to be interpreted as described in 195 [RFC2119]. 197 2. The Service Interface for Requesting IP Multicast Reception 199 Within an IP system, there is (at least conceptually) a service 200 interface used by upper-layer protocols or application programs to 201 ask the IP layer to enable and disable reception of packets sent to 202 specific IP multicast addresses. In order to take full advantage of 203 the capabilities of IGMPv3, a system's IP service interface must 204 support the following operation: 206 IPMulticastListen ( socket, interface, multicast-address, 207 filter-mode, source-list ) 209 where: 211 o "socket" is an implementation-specific parameter used to 212 distinguish among different requesting entities (e.g., programs or 213 processes) within the system; the socket parameter of BSD Unix 214 system calls is a specific example. 216 o "interface" is a local identifier of the network interface on 217 which reception of the specified multicast address is to be 218 enabled or disabled. Interfaces may be physical (e.g., an 219 Ethernet interface) or virtual (e.g., the endpoint of a Frame 220 Relay virtual circuit or the endpoint of an IP-in-IP "tunnel"). 221 An implementation may allow a special "unspecified" value to be 222 passed as the interface parameter, in which case the request would 223 apply to the "primary" or "default" interface of the system 224 (perhaps established by system configuration). If reception of 225 the same multicast address is desired on more than one interface, 226 IPMulticastListen is invoked separately for each desired 227 interface. 229 o "multicast-address" is the IP multicast address, or group, to 230 which the request pertains. If reception of more than one 231 multicast address on a given interface is desired, 232 IPMulticastListen is invoked separately for each desired multicast 233 address. 235 o "filter-mode" may be either INCLUDE or EXCLUDE. In INCLUDE mode, 236 reception of packets sent to the specified multicast address is 237 requested only from those IP source addresses listed in the 238 source-list parameter. In EXCLUDE mode, reception of packets sent 239 to the given multicast address is requested from all IP source 240 addresses except those listed in the source-list parameter. 242 o "source-list" is an unordered list of zero or more IP unicast 243 addresses from which multicast reception is desired or not 244 desired, depending on the filter mode. An implementation MAY 245 impose a limit on the size of source lists, but that limit MUST 246 NOT be less than 64 addresses per list. When an operation causes 247 the source list size limit to be exceeded, the service interface 248 MUST return an error. 250 For a given combination of socket, interface, and multicast address, 251 only a single filter mode and source list can be in effect at any one 252 time. However, either the filter mode or the source list, or both, 253 may be changed by subsequent IPMulticastListen requests that specify 254 the same socket, interface, and multicast address. Each subsequent 255 request completely replaces any earlier request for the given socket, 256 interface and multicast address. 258 Previous versions of IGMP did not support source filters and had a 259 simpler service interface consisting of Join and Leave operations to 260 enable and disable reception of a given multicast address (from all 261 sources) on a given interface. The equivalent operations in the new 262 service interface follow: 264 The Join operation is equivalent to: 266 IPMulticastListen ( socket, interface, multicast-address, 267 EXCLUDE, {} ) 269 and the Leave operation is equivalent to: 271 IPMulticastListen ( socket, interface, multicast-address, 272 INCLUDE, {} ) 274 where {} is an empty source list. 276 An example of an API providing the capabilities outlined in this 277 service interface is in [RFC3678]. 279 3. Multicast Reception State Maintained by Systems 281 3.1. Socket State 283 For each socket on which IPMulticastListen has been invoked, the 284 system records the desired multicast reception state for that socket. 285 That state conceptually consists of a set of records of the form: 287 (interface, multicast-address, filter-mode, source-list) 289 The socket state evolves in response to each invocation of 290 IPMulticastListen on the socket, as follows: 292 o If the requested filter mode is INCLUDE and the requested source 293 list is empty, then the entry corresponding to the requested 294 interface and multicast address is deleted if present. If no such 295 entry is present, the request is ignored. 297 o If the requested filter mode is EXCLUDE or the requested source 298 list is non-empty, then the entry corresponding to the requested 299 interface and multicast address, if present, is changed to contain 300 the requested filter mode and source list. If no such entry is 301 present, a new entry is created, using the parameters specified in 302 the request. 304 3.2. Interface State 306 In addition to the per-socket multicast reception state, a system 307 must also maintain or compute multicast reception state for each of 308 its interfaces. That state conceptually consists of a set of records 309 of the form: 311 (multicast-address, filter-mode, source-list) 313 At most one record per multicast-address exists for a given 314 interface. This per-interface state is derived from the per-socket 315 state, but may differ from the per-socket state when different 316 sockets have differing filter modes and/or source lists for the same 317 multicast address and interface. For example, suppose one 318 application or process invokes the following operation on socket s1: 320 IPMulticastListen ( s1, i, m, INCLUDE, {a, b, c} ) 322 requesting reception on interface i of packets sent to multicast 323 address m, only if they come from source a, b, or c. Suppose another 324 application or process invokes the following operation on socket s2: 326 IPMulticastListen ( s2, i, m, INCLUDE, {b, c, d} ) 328 requesting reception on the same interface i of packets sent to the 329 same multicast address m, only if they come from sources b, c, or d. 330 In order to satisfy the reception requirements of both sockets, it is 331 necessary for interface i to receive packets sent to m from any one 332 of the sources a, b, c, or d. Thus, in this example, the reception 333 state of interface i for multicast address m has filter mode INCLUDE 334 and source list {a, b, c, d}. 336 After a multicast packet has been accepted from an interface by the 337 IP layer, its subsequent delivery to the application or process 338 listening on a particular socket depends on the multicast reception 339 state of that socket [and possibly also on other conditions, such as 340 what transport-layer port the socket is bound to]. So, in the above 341 example, if a packet arrives on interface i, destined to multicast 342 address m, with source address a, it will be delivered on socket s1 343 but not on socket s2. Note that IGMP Queries and Reports are not 344 subject to source filtering and must always be processed by hosts and 345 routers. 347 Filtering of packets based upon a socket's multicast reception state 348 is a new feature of this service interface. The previous service 349 interface [RFC1112] described no filtering based upon multicast join 350 state; rather, a join on a socket simply caused the host to join a 351 group on the given interface, and packets destined for that group 352 could be delivered to all sockets whether they had joined or not. 354 The general rules for deriving the per-interface state from the per- 355 socket state are as follows: For each distinct (interface, multicast- 356 address) pair that appears in any socket state, a per- interface 357 record is created for that multicast address on that interface. 358 Considering all socket records containing the same (interface, 359 multicast-address) pair, 361 o if any such record has a filter mode of EXCLUDE, then the filter 362 mode of the interface record is EXCLUDE, and the source list of 363 the interface record is the intersection of the source lists of 364 all socket records in EXCLUDE mode, minus those source addresses 365 that appear in any socket record in INCLUDE mode. For example, if 366 the socket records for multicast address m on interface i are: 368 from socket s1: ( i, m, EXCLUDE, {a, b, c, d} ) 370 from socket s2: ( i, m, EXCLUDE, {b, c, d, e} ) 372 from socket s3: ( i, m, INCLUDE, {d, e, f} ) 374 then the corresponding interface record on interface i is: 376 ( m, EXCLUDE, {b, c} ) 378 If a fourth socket is added, such as: 380 from socket s4: ( i, m, EXCLUDE, {} ) 382 then the interface record becomes: 384 ( m, EXCLUDE, {} ) 386 o if all such records have a filter mode of INCLUDE, then the filter 387 mode of the interface record is INCLUDE, and the source list of 388 the interface record is the union of the source lists of all the 389 socket records. For example, if the socket records for multicast 390 address m on interface i are: 392 from socket s1: ( i, m, INCLUDE, {a, b, c} ) 394 from socket s2: ( i, m, INCLUDE, {b, c, d} ) 396 from socket s3: ( i, m, INCLUDE, {e, f} ) 398 then the corresponding interface record on interface i is: 400 ( m, INCLUDE, {a, b, c, d, e, f} ) 402 An implementation MUST NOT use an EXCLUDE interface record to 403 represent a group when all sockets for this group are in INCLUDE 404 state. If system resource limits are reached when an interface 405 state source list is calculated, an error MUST be returned to the 406 application which requested the operation. 408 The above rules for deriving the interface state are (re-)evaluated 409 whenever an IPMulticastListen invocation modifies the socket state by 410 adding, deleting, or modifying a per-socket state record. Note that 411 a change of socket state does not necessarily result in a change of 412 interface state. 414 4. Message Formats 416 IGMP messages are encapsulated in IPv4 datagrams, with an IP protocol 417 number of 2. Every IGMP message described in this document is sent 418 with an IP Time-to-Live of 1, IP Precedence of Internetwork Control 419 (e.g., Type of Service 0xc0), and carries an IP Router Alert option 420 [RFC2113] in its IP header. IGMP message types are registered per 421 [RFC3228]. 423 There are two IGMP message types of concern to the IGMPv3 protocol 424 described in this document: 426 +-------------------+-----------------------------+ 427 | Type Number (hex) | Message Name | 428 +-------------------+-----------------------------+ 429 | 0x11 | Membership Query | 430 | 0x22 | Version 3 Membership Report | 431 +-------------------+-----------------------------+ 433 Table 1: New messages introduced by IGMP3 435 An implementation of IGMPv3 MUST also support the following three 436 message types, for interoperation with previous versions of IGMP (see 437 Section 7): 439 +-------------------+-----------------------------+-----------+ 440 | Type Number (hex) | Message Name | Reference | 441 +-------------------+-----------------------------+-----------+ 442 | 0x12 | Version 1 Membership Report | [RFC1112] | 443 | 0x16 | Version 2 Membership Report | [RFC2236] | 444 | 0x17 | Version 2 Leave Group | [RFC2236] | 445 +-------------------+-----------------------------+-----------+ 447 Table 2: Legacy IGMP messages 449 Unrecognized message types MUST be silently ignored. Other message 450 types may be used by newer versions or extensions of IGMP, by 451 multicast routing protocols, or for other uses. 453 In this document, unless otherwise qualified, the capitalized words 454 "Query" and "Report" refer to IGMP Membership Queries and IGMP 455 Version 3 Membership Reports, respectively. 457 4.1. Membership Query Message 459 Membership Queries are sent by IP multicast routers to query the 460 multicast reception state of neighboring interfaces. Queries have 461 the following format: 463 0 1 2 3 464 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 465 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 466 | Type = 0x11 | Max Resp Code | Checksum | 467 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 468 | Group Address | 469 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 470 | Resv |S| QRV | QQIC | Number of Sources (N) | 471 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 472 | Source Address [1] | 473 +- -+ 474 | Source Address [2] | 475 +- . -+ 476 . . . 477 . . . 478 +- -+ 479 | Source Address [N] | 480 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 482 Figure 1: IGMPv3 Query Message 484 4.1.1. Max Resp Code 486 The Max Resp Code field specifies the maximum time allowed before 487 sending a responding report. The actual time allowed, called the Max 488 Resp Time, is represented in units of 1/10 second and is derived from 489 the Max Resp Code as follows: 491 If Max Resp Code < 128, Max Resp Time = Max Resp Code 493 If Max Resp Code >= 128, Max Resp Code represents a floating-point 494 value as follows: 496 0 1 2 3 4 5 6 7 497 +-+-+-+-+-+-+-+-+ 498 |1| exp | mant | 499 +-+-+-+-+-+-+-+-+ 501 Max Resp Time = (mant | 0x10) << (exp + 3) 503 Figure 2: Max Resp Code Representation 505 Small values of Max Resp Time allow IGMPv3 routers to tune the "leave 506 latency" (the time between the moment the last host leaves a group 507 and the moment the routing protocol is notified that there are no 508 more members). Larger values, especially in the exponential range, 509 allow tuning of the burstiness of IGMP traffic on a network. 511 4.1.2. Checksum 513 The Checksum is the 16-bit one's complement of the one's complement 514 sum of the whole IGMP message (the entire IP payload). For computing 515 the checksum, the Checksum field is set to zero. When receiving 516 packets, the checksum MUST be verified before processing a packet 517 [RFC1071]. 519 4.1.3. Group Address 521 The Group Address field is set to zero when sending a General Query, 522 and set to the IP multicast address being queried when sending a 523 Group-Specific Query or Group-and-Source-Specific Query (see 524 Section Section 4.1.9, below). 526 4.1.4. Resv (Reserved) 528 The Resv field is set to zero on transmission, and ignored on 529 reception. 531 4.1.5. S Flag (Suppress Router-Side Processing) 533 When set to one, the S Flag indicates to any receiving multicast 534 routers that they are to suppress the normal timer updates they 535 perform upon hearing a Query. It does not, however, suppress the 536 querier election or the normal "host-side" processing of a Query that 537 a router may be required to perform as a consequence of itself being 538 a group member. 540 4.1.6. QRV (Querier's Robustness Variable) 542 If non-zero, the QRV field contains the [Robustness Variable] value 543 used by the querier, i.e., the sender of the Query. If the querier's 544 [Robustness Variable] exceeds 7, the maximum value of the QRV field, 545 the QRV is set to zero. Routers adopt the QRV value from the most 546 recently received Query as their own [Robustness Variable] value, 547 unless that most recently received QRV was zero, in which case the 548 receivers use the default [Robustness Variable] value specified in 549 section Section 8.1 or a statically configured value. 551 4.1.7. QQIC (Querier's Query Interval Code) 553 The Querier's Query Interval Code field specifies the [Query 554 Interval] used by the querier. The actual interval, called the 555 Querier's Query Interval (QQI), is represented in units of seconds 556 and is derived from the Querier's Query Interval Code as follows: 558 If QQIC < 128, QQI = QQIC 559 If QQIC >= 128, QQIC represents a floating-point value as follows: 561 0 1 2 3 4 5 6 7 562 +-+-+-+-+-+-+-+-+ 563 |1| exp | mant | 564 +-+-+-+-+-+-+-+-+ 566 QQI = (mant | 0x10) << (exp + 3) 568 Figure 3: QQIC Representation 570 Multicast routers that are not the current querier adopt the QQI 571 value from the most recently received Query as their own [Query 572 Interval] value, unless that most recently received QQI was zero, in 573 which case the receiving routers use the default [Query Interval] 574 value specified in Section 8.2. 576 4.1.8. Number of Sources (N) 578 The Number of Sources (N) field specifies how many source addresses 579 are present in the Query. This number is zero in a General Query or 580 a Group-Specific Query, and non-zero in a Group-and-Source-Specific 581 Query. This number is limited by the MTU of the network over which 582 the Query is transmitted. For example, on an Ethernet with an MTU of 583 1500 octets, the IP header including the Router Alert option consumes 584 24 octets, and the IGMP fields up to including the Number of Sources 585 (N) field consume 12 octets, leaving 1464 octets for source 586 addresses, which limits the number of source addresses to 366 587 (1464/4). 589 4.1.9. Source Address [i] 591 The Source Address [i] fields are a vector of n IP unicast addresses, 592 where n is the value in the Number of Sources (N) field. 594 4.1.10. Additional Data 596 If the Packet Length field in the IP header of a received Query 597 indicates that there are additional octets of data present, beyond 598 the fields described here, IGMPv3 implementations MUST include those 599 octets in the computation to verify the received IGMP Checksum, but 600 MUST otherwise ignore those additional octets. When sending a Query, 601 an IGMPv3 implementation MUST NOT include additional octets beyond 602 the fields described here. 604 4.1.11. Query Variants 606 There are three variants of the Query message: 608 1. A General Query is sent by a multicast router to learn the 609 complete multicast reception state of the neighboring interfaces 610 (that is, the interfaces attached to the network on which the 611 Query is transmitted). In a General Query, both the Group 612 Address field and the Number of Sources (N) field are zero. 614 2. A Group-Specific Query is sent by a multicast router to learn the 615 reception state, with respect to a single multicast address, of 616 the neighboring interfaces. In a Group-Specific Query, the Group 617 Address field contains the multicast address of interest, and the 618 Number of Sources (N) field contains zero. 620 3. A Group-and-Source-Specific Query is sent by a multicast router 621 to learn if any neighboring interface desires reception of 622 packets sent to a specified multicast address, from any of a 623 specified list of sources. In a Group-and-Source-Specific Query, 624 the Group Address field contains the multicast address of 625 interest, and the Source Address [i] fields contain the source 626 address(es) of interest. 628 4.1.12. IP Destination Addresses for Queries 630 In IGMPv3, General Queries are sent with an IP destination address of 631 224.0.0.1, the all-systems multicast address. Group-Specific and 632 Group-and-Source-Specific Queries are sent with an IP destination 633 address equal to the multicast address of interest. However, a 634 system MUST accept and process any Query whose IP Destination Address 635 field contains any of the addresses (unicast or multicast) assigned 636 to the interface on which the Query arrives. 638 4.2. Version 3 Membership Report Message 640 Version 3 Membership Reports are sent by IP systems to report (to 641 neighboring routers) the current multicast reception state, or 642 changes in the multicast reception state, of their interfaces. 643 Reports have the following format: 645 0 1 2 3 646 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 647 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 648 | Type = 0x22 | Reserved | Checksum | 649 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 650 | Reserved | Number of Group Records (M) | 651 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 652 | | 653 . . 654 . Group Record [1] . 655 . . 656 | | 657 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 658 | | 659 . . 660 . Group Record [2] . 661 . . 662 | | 663 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 664 | . | 665 . . . 666 | . | 667 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 668 | | 669 . . 670 . Group Record [M] . 671 . . 672 | | 673 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 675 Figure 4: IGMPv3 Report Message 677 where each Group Record has the following internal format: 679 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 680 | Record Type | Aux Data Len | Number of Sources (N) | 681 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 682 | Multicast Address | 683 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 684 | Source Address [1] | 685 +- -+ 686 | Source Address [2] | 687 +- -+ 688 . . . 689 . . . 690 . . . 691 +- -+ 692 | Source Address [N] | 693 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 694 | | 695 . . 696 . Auxiliary Data . 697 . . 698 | | 699 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 701 Figure 5: IGMPv3 Report Group Record 703 4.2.1. Reserved 705 The Reserved fields are set to zero on transmission, and ignored on 706 reception. 708 4.2.2. Checksum 710 The Checksum is the 16-bit one's complement of the one's complement 711 sum of the whole IGMP message (the entire IP payload). For computing 712 the checksum, the Checksum field is set to zero. When receiving 713 packets, the checksum MUST be verified before processing a message. 715 4.2.3. Number of Group Records (M) 717 The Number of Group Records (M) field specifies how many Group 718 Records are present in this Report. 720 4.2.4. Group Record 722 Each Group Record is a block of fields containing information 723 pertaining to the sender's membership in a single multicast group on 724 the interface from which the Report is sent. 726 4.2.5. Record Type 728 See section Section 4.2.12, below. 730 4.2.6. Aux Data Len 732 The Aux Data Len field contains the length of the Auxiliary Data 733 field in this Group Record, in units of 32-bit words. It may contain 734 zero, to indicate the absence of any auxiliary data. 736 4.2.7. Number of Sources (N) 738 The Number of Sources (N) field specifies how many source addresses 739 are present in this Group Record. 741 4.2.8. Multicast Address 743 The Multicast Address field contains the IP multicast address to 744 which this Group Record pertains. 746 4.2.9. Source Address [i] 748 The Source Address [i] fields are a vector of n IP unicast addresses, 749 where n is the value in this record's Number of Sources (N) field. 751 4.2.10. Auxiliary Data 753 The Auxiliary Data field, if present, contains additional information 754 pertaining to this Group Record. The protocol specified in this 755 document, IGMPv3, does not define any auxiliary data. Therefore, 756 implementations of IGMPv3 MUST NOT include any auxiliary data (i.e., 757 MUST set the Aux Data Len field to zero) in any transmitted Group 758 Record, and MUST ignore any auxiliary data present in any received 759 Group Record. The semantics and internal encoding of the Auxiliary 760 Data field are to be defined by any future version or extension of 761 IGMP that uses this field. 763 4.2.11. Additional Data 765 If the Packet Length field in the IP header of a received Report 766 indicates that there are additional octets of data present, beyond 767 the last Group Record, IGMPv3 implementations MUST include those 768 octets in the computation to verify the received IGMP Checksum, but 769 MUST otherwise ignore those additional octets. When sending a 770 Report, an IGMPv3 implementation MUST NOT include additional octets 771 beyond the last Group Record. 773 4.2.12. Group Record Types 775 There are a number of different types of Group Records that may be 776 included in a Report message: 778 o A Current-State Record is sent by a system in response to a Query 779 received on an interface. It reports the current reception state 780 of that interface, with respect to a single multicast address. 781 The Record Type of a Current-State Record may be one of the 782 following two values: 784 1 - MODE_IS_INCLUDE - indicates that the interface has a filter 785 mode of INCLUDE for the specified multicast address. The 786 Source Address [i] fields in this Group Record contain the 787 interface's source list for the specified multicast address, 788 if it is non-empty. 790 2 - MODE_IS_EXCLUDE - indicates that the interface has a filter 791 mode of EXCLUDE for the specified multicast address. The 792 Source Address [i] fields in this Group Record contain the 793 interface's source list for the specified multicast address, 794 if it is non-empty. 796 o A Filter-Mode-Change Record is sent by a system whenever a local 797 invocation of IPMulticastListen causes a change of the filter mode 798 (i.e., a change from INCLUDE to EXCLUDE, or from EXCLUDE to 799 INCLUDE), of the interface-level state entry for a particular 800 multicast address. The Record is included in a Report sent from 801 the interface on which the change occurred. The Record Type of a 802 Filter-Mode-Change Record may be one of the following two values: 804 3 - CHANGE_TO_INCLUDE_MODE - indicates that the interface has 805 changed to INCLUDE filter mode for the specified multicast 806 address. The Source Address [i] fields in this Group Record 807 contain the interface's new source list for the specified 808 multicast address, if it is non-empty. 810 4 - CHANGE_TO_EXCLUDE_MODE - indicates that the interface has 811 changed to EXCLUDE filter mode for the specified multicast 812 address. The Source Address [i] fields in this Group Record 813 contain the interface's new source list for the specified 814 multicast address, if it is non-empty. 816 o A Source-List-Change Record is sent by a system whenever a local 817 invocation of IPMulticastListen causes a change of source list 818 that is not coincident with a change of filter mode, of the 819 interface-level state entry for a particular multicast address. 820 The Record is included in a Report sent from the interface on 821 which the change occurred. The Record Type of a Source-List- 822 Change Record may be one of the following two values: 824 5 - ALLOW_NEW_SOURCES - indicates that the Source Address [i] 825 fields in this Group Record contain a list of the additional 826 sources that the system wishes to hear from, for packets 827 sent to the specified multicast address. If the change was 828 to an INCLUDE source list, these are the addresses that were 829 added to the list; if the change was to an EXCLUDE source 830 list, these are the addresses that were deleted from the 831 list. 833 6 - BLOCK_OLD_SOURCES - indicates that the Source Address [i] 834 fields in this Group Record contain a list of the sources 835 that the system no longer wishes to hear from, for packets 836 sent to the specified multicast address. If the change was 837 to an INCLUDE source list, these are the addresses that were 838 deleted from the list; if the change was to an EXCLUDE 839 source list, these are the addresses that were added to the 840 list. 842 If a change of source list results in both allowing new sources and 843 blocking old sources, then two Group Records are sent for the same 844 multicast address, one of type ALLOW_NEW_SOURCES and one of type 845 BLOCK_OLD_SOURCES. 847 We use the term State-Change Record to refer to either a Filter- 848 Mode-Change Record or a Source-List-Change Record. 850 Unrecognized Record Type values MUST be silently ignored. 852 4.2.13. IP Source Addresses for Reports 854 An IGMP report is sent with a valid IP source address for the 855 destination subnet. The 0.0.0.0 source address may be used by a 856 system that has not yet acquired an IP address. Note that the 857 0.0.0.0 source address may simultaneously be used by multiple systems 858 on a LAN. Routers MUST accept a report with a source address of 859 0.0.0.0. 861 4.2.14. IP Destination Addresses for Reports 863 Version 3 Reports are sent with an IP destination address of 864 224.0.0.22, to which all IGMPv3-capable multicast routers listen. A 865 system that is operating in version 1 or version 2 compatibility 866 modes sends version 1 or version 2 Reports to the multicast group 867 specified in the Group Address field of the Report. In addition, a 868 system MUST accept and process any version 1 or version 2 Report 869 whose IP Destination Address field contains any of the addresses 870 (unicast or multicast) assigned to the interface on which the Report 871 arrives. 873 4.2.15. Notation for Group Records 875 In the rest of this document, we use the following notation to 876 describe the contents of a Group Record pertaining to a particular 877 multicast address: 879 IS_IN ( x ) - Type MODE_IS_INCLUDE, source addresses x 880 IS_EX ( x ) - Type MODE_IS_EXCLUDE, source addresses x 881 TO_IN ( x ) - Type CHANGE_TO_INCLUDE_MODE, source addresses x 882 TO_EX ( x ) - Type CHANGE_TO_EXCLUDE_MODE, source addresses x 883 ALLOW ( x ) - Type ALLOW_NEW_SOURCES, source addresses x 884 BLOCK ( x ) - Type BLOCK_OLD_SOURCES, source addresses x 886 where x is either: 888 o a capital letter (e.g., "A") to represent the set of source 889 addresses, or 891 o a set expression (e.g., "A+B"), where "A+B" means the union of 892 sets A and B, "A*B" means the intersection of sets A and B, and 893 "A-B" means the removal of all elements of set B from set A. 895 4.2.16. Membership Report Size 897 If the set of Group Records required in a Report does not fit within 898 the size limit of a single Report message (as determined by the MTU 899 of the network on which it will be sent), the Group Records are sent 900 in as many Report messages as needed to report the entire set. 902 If a single Group Record contains so many source addresses that it 903 does not fit within the size limit of a single Report message, if its 904 Type is not MODE_IS_EXCLUDE or CHANGE_TO_EXCLUDE_MODE, it is split 905 into multiple Group Records, each containing a different subset of 906 the source addresses and each sent in a separate Report message. If 907 its Type is MODE_IS_EXCLUDE or CHANGE_TO_EXCLUDE_MODE, a single Group 908 Record is sent, containing as many source addresses as can fit, and 910 the remaining source addresses are not reported; though the choice of 911 which sources to report is arbitrary, it is preferable to report the 912 same set of sources in each subsequent report, rather than reporting 913 different sources each time. 915 5. Description of the Protocol for Group Members 917 IGMP is an asymmetric protocol, specifying separate behaviors for 918 group members -- that is, hosts or routers that wish to receive 919 multicast packets -- and multicast routers. This section describes 920 the part of IGMPv3 that applies to all group members. (Note that a 921 multicast router that is also a group member performs both parts of 922 IGMPv3, receiving and responding to its own IGMP message 923 transmissions as well as those of its neighbors. The multicast 924 router part of IGMPv3 is described in Section 6.) 926 A system performs the protocol described in this section over all 927 interfaces on which multicast reception is supported, even if more 928 than one of those interfaces is connected to the same network. 930 For interoperability with multicast routers running older versions of 931 IGMP, systems maintain a MulticastRouterVersion variable for each 932 interface on which multicast reception is supported. This section 933 describes the behavior of group member systems on interfaces for 934 which MulticastRouterVersion = 3. The algorithm for determining 935 MulticastRouterVersion, and the behavior for versions other than 3, 936 are described in Section 7. 938 The all-systems multicast address, 224.0.0.1, is handled as a special 939 case. On all systems -- that is all hosts and routers, including 940 multicast routers -- reception of packets destined to the all-systems 941 multicast address, from all sources, is permanently enabled on all 942 interfaces on which multicast reception is supported. No IGMP 943 messages are ever sent regarding the all-systems multicast address. 945 There are two types of events that trigger IGMPv3 protocol actions on 946 an interface: 948 o a change of the interface reception state, caused by a local 949 invocation of IPMulticastListen. 951 o reception of a Query. 953 (Received IGMP messages of types other than Query are silently 954 ignored, except as required for interoperation with earlier versions 955 of IGMP.) 957 The following subsections describe the actions to be taken for each 958 of these two cases. In those descriptions, timer and counter names 959 appear in square brackets. The default values for those timers and 960 counters are specified in Section 8. 962 5.1. Action on Change of Interface State 964 An invocation of IPMulticastListen may cause the multicast reception 965 state of an interface to change, according to the rules in 966 Section Section 3.2. Each such change affects the per-interface 967 entry for a single multicast address. 969 A change of interface state causes the system to immediately transmit 970 a State-Change Report from that interface. The type and contents of 971 the Group Record(s) in that Report are determined by comparing the 972 filter mode and source list for the affected multicast address before 973 and after the change, according to the table below. If no interface 974 state existed for that multicast address before the change (i.e., the 975 change consisted of creating a new per-interface record), or if no 976 state exists after the change (i.e., the change consisted of deleting 977 a per-interface record), then the "non-existent" state is considered 978 to have a filter mode of INCLUDE and an empty source list. 980 +-------------+-------------+--------------------------+ 981 | Old State | New State | State-Change Record Sent | 982 +-------------+-------------+--------------------------+ 983 | INCLUDE (A) | INCLUDE (B) | ALLOW (B-A), BLOCK (A-B) | 984 | EXCLUDE (A) | EXCLUDE (B) | ALLOW (A-B), BLOCK (B-A) | 985 | INCLUDE (A) | EXCLUDE (B) | TO_EX (B) | 986 | EXCLUDE (A) | INCLUDE (B) | TO_IN (B) | 987 +-------------+-------------+--------------------------+ 989 If the computed source list for either an ALLOW or a BLOCK State- 990 Change Record is empty, that record is omitted from the Report 991 message. 993 To cover the possibility of the State-Change Report being missed by 994 one or more multicast routers, it is retransmitted [Robustness 995 Variable] - 1 more times, at intervals chosen at random from the 996 range (0, [Unsolicited Report Interval]). 998 If more changes to the same interface state entry occur before all 999 the retransmissions of the State-Change Report for the first change 1000 have been completed, each such additional change triggers the 1001 immediate transmission of a new State-Change Report. 1003 The contents of the new transmitted report are calculated as follows. 1004 As was done with the first report, the interface state for the 1005 affected group before and after the latest change is compared. The 1006 report records expressing the difference are built according to the 1007 table above. However these records are not transmitted in a message 1008 but instead merged with the contents of the pending report, to create 1009 the new State-Change report. The rules for merging the difference 1010 report resulting from the state change and the pending report are 1011 described below. 1013 The transmission of the merged State-Change Report terminates 1014 retransmissions of the earlier State-Change Reports for the same 1015 multicast address, and becomes the first of [Robustness Variable] 1016 transmissions of State-Change Reports. 1018 Each time a source is included in the difference report calculated 1019 above, retransmission state for that source needs to be maintained 1020 until [Robustness Variable] State-Change reports have been sent by 1021 the host. This is done in order to ensure that a series of 1022 successive state changes do not break the protocol robustness. 1024 If the interface reception-state change that triggers the new report 1025 is a filter-mode change, then the next [Robustness Variable] State- 1026 Change Reports will include a Filter-Mode-Change record. This 1027 applies even if any number of source-list changes occur in that 1028 period. The host has to maintain retransmission state for the group 1029 until the [Robustness Variable] State-Change reports have been sent. 1030 When [Robustness Variable] State-Change reports with Filter-Mode- 1031 Change records have been transmitted after the last filter-mode 1032 change, and if source-list changes to the interface reception have 1033 scheduled additional reports, then the next State-Change report will 1034 include Source-List-Change records. 1036 Each time a State-Change Report is transmitted, the contents are 1037 determined as follows. If the report should contain a Filter-Mode- 1038 Change record, then if the current filter-mode of the interface is 1039 INCLUDE, a TO_IN record is included in the report, otherwise a TO_EX 1040 record is included. If instead the report should contain Source- 1041 List-Change records, an ALLOW and a BLOCK record are included. The 1042 contents of these records are built according to the table below. 1044 +--------+----------------------------------------------------------+ 1045 | Record | Sources Included | 1046 +--------+----------------------------------------------------------+ 1047 | TO_IN | All in the current interface state that must be | 1048 | | forwarded | 1049 | TO_EX | All in the current interface state that must be blocked | 1050 | ALLOW | All with retransmission state that must be forwarded | 1051 | BLOCK | All with retransmission state that must be blocked | 1052 +--------+----------------------------------------------------------+ 1054 If the computed source list for either an ALLOW or a BLOCK record is 1055 empty, that record is omitted from the State-Change report. 1057 Note: When the first State-Change report is sent, the non-existent 1058 pending report to merge with, can be treated as a source-change 1059 report with empty ALLOW and BLOCK records (no sources have 1060 retransmission state). 1062 5.2. Action on Reception of a Query 1064 When a system receives a Query, it does not respond immediately. 1065 Instead, it delays its response by a random amount of time, bounded 1066 by the Max Resp Time value derived from the Max Resp Code in the 1067 received Query message. A system may receive a variety of Queries on 1068 different interfaces and of different kinds (e.g., General Queries, 1069 Group-Specific Queries, and Group-and-Source-Specific Queries), each 1070 of which may require its own delayed response. 1072 Before scheduling a response to a Query, the system must first 1073 consider previously scheduled pending responses and in many cases 1074 schedule a combined response. Therefore, the system must be able to 1075 maintain the following state: 1077 o A timer per interface for scheduling responses to General Queries. 1079 o A per-group and interface timer for scheduling responses to Group- 1080 Specific and Group-and-Source-Specific Queries. 1082 o A per-group and interface list of sources to be reported in the 1083 response to a Group-and-Source-Specific Query. 1085 When a new Query with the Router-Alert option arrives on an 1086 interface, provided the system has state to report, a delay for a 1087 response is randomly selected in the range (0, [Max Resp Time]) where 1088 Max Resp Time is derived from Max Resp Code in the received Query 1089 message. The following rules are then used to determine if a Report 1090 needs to be scheduled and the type of Report to schedule. The rules 1091 are considered in order and only the first matching rule is applied. 1093 1. If there is a pending response to a previous General Query 1094 scheduled sooner than the selected delay, no additional response 1095 needs to be scheduled. 1097 2. If the received Query is a General Query, the interface timer is 1098 used to schedule a response to the General Query after the 1099 selected delay. Any previously pending response to a General 1100 Query is canceled. 1102 3. If the received Query is a Group-Specific Query or a Group-and- 1103 Source-Specific Query and there is no pending response to a 1104 previous Query for this group, then the group timer is used to 1105 schedule a report. If the received Query is a Group-and-Source- 1106 Specific Query, the list of queried sources is recorded to be 1107 used when generating a response. 1109 4. If there already is a pending response to a previous Query 1110 scheduled for this group, and either the new Query is a Group- 1111 Specific Query or the recorded source-list associated with the 1112 group is empty, then the group source-list is cleared and a 1113 single response is scheduled using the group timer. The new 1114 response is scheduled to be sent at the earliest of the remaining 1115 time for the pending report and the selected delay. 1117 5. If the received Query is a Group-and-Source-Specific Query and 1118 there is a pending response for this group with a non-empty 1119 source-list, then the group source list is augmented to contain 1120 the list of sources in the new Query and a single response is 1121 scheduled using the group timer. The new response is scheduled 1122 to be sent at the earliest of the remaining time for the pending 1123 report and the selected delay. 1125 When the timer in a pending response record expires, the system 1126 transmits, on the associated interface, one or more Report messages 1127 carrying one or more Current-State Records (see section 1128 Section 4.2.12), as follows: 1130 1. If the expired timer is the interface timer (i.e., it is a 1131 pending response to a General Query), then one Current-State 1132 Record is sent for each multicast address for which the specified 1133 interface has reception state, as described in Section 3.2. The 1134 Current- State Record carries the multicast address and its 1135 associated filter mode (MODE_IS_INCLUDE or MODE_IS_EXCLUDE) and 1136 source list. Multiple Current-State Records are packed into 1137 individual Report messages, to the extent possible. 1139 This naive algorithm may result in bursts of packets when a 1140 system is a member of a large number of groups. Instead of using 1141 a single interface timer, implementations are recommended to 1142 spread transmission of such Report messages over the interval (0, 1143 [Max Resp Time]). Note that any such implementation MUST avoid 1144 the "ack-implosion" problem, i.e., MUST NOT send a Report 1145 immediately on reception of a General Query. 1147 2. If the expired timer is a group timer and the list of recorded 1148 sources for the that group is empty (i.e., it is a pending 1149 response to a Group-Specific Query), then if and only if the 1150 interface has reception state for that group address, a single 1151 Current-State Record is sent for that address. The Current-State 1152 Record carries the multicast address and its associated filter 1153 mode (MODE_IS_INCLUDE or MODE_IS_EXCLUDE) and source list. 1155 3. If the expired timer is a group timer and the list of recorded 1156 sources for that group is non-empty (i.e., it is a pending 1157 response to a Group-and-Source-Specific Query), then if and only 1158 if the interface has reception state for that group address, the 1159 contents of the responding Current-State Record is determined 1160 from the interface state and the pending response record, as 1161 specified in the following table: 1163 +--------------+----------------------------------+-----------------+ 1164 | interface | set of sources in the pending | Current-State | 1165 | state | response record | Record | 1166 +--------------+----------------------------------+-----------------+ 1167 | INCLUDE (A) | B | IS_IN (A*B) | 1168 | EXCLUDE (A) | B | IS_IN (B-A) | 1169 +--------------+----------------------------------+-----------------+ 1171 If the resulting Current-State Record has an empty set of source 1172 addresses, then no response is sent. 1174 Finally, after any required Report messages have been generated, the 1175 source lists associated with any reported groups are cleared. 1177 6. Description of the Protocol for Multicast Routers 1179 The purpose of IGMP is to enable each multicast router to learn, for 1180 each of its directly attached networks, which multicast addresses are 1181 of interest to the systems attached to those networks. IGMP version 1182 3 adds the capability for a multicast router to also learn which 1183 sources are of interest to neighboring systems, for packets sent to 1184 any particular multicast address. The information gathered by IGMP 1185 is provided to whichever multicast routing protocol is being used by 1186 the router, in order to ensure that multicast packets are delivered 1187 to all networks where there are interested receivers. 1189 This section describes the part of IGMPv3 that is performed by 1190 multicast routers. Multicast routers may also themselves become 1191 members of multicast groups, and therefore also perform the group 1192 member part of IGMPv3, described in Section 5. 1194 A multicast router performs the protocol described in this section 1195 over each of its directly-attached networks. If a multicast router 1196 has more than one interface to the same network, it only needs to 1197 operate this protocol over one of those interfaces. On each 1198 interface over which this protocol is being run, the router MUST 1199 enable reception of multicast address 224.0.0.22, from all sources 1200 (and MUST perform the group member part of IGMPv3 for that address on 1201 that interface). 1203 Multicast routers need to know only that at least one system on an 1204 attached network is interested in packets to a particular multicast 1205 address from a particular source; a multicast router is not required 1206 to keep track of the interests of each individual neighboring system. 1207 (However, see Appendix A.2 point 1 for discussion.) 1209 IGMPv3 is backward compatible with previous versions of the IGMP 1210 protocol. In order to remain backward compatible with older IGMP 1211 systems, IGMPv3 multicast routers MUST also implement versions 1 and 1212 2 of the protocol (see section Section 7). 1214 6.1. Conditions for IGMP Queries 1216 Multicast routers send General Queries periodically to request group 1217 membership information from an attached network. These queries are 1218 used to build and refresh the group membership state of systems on 1219 attached networks. Systems respond to these queries by reporting 1220 their group membership state (and their desired set of sources) with 1221 Current-State Group Records in IGMPv3 Membership Reports. 1223 As a member of a multicast group, a system may express interest in 1224 receiving or not receiving traffic from particular sources. As the 1225 desired reception state of a system changes, it reports these changes 1226 using Filter-Mode-Change Records or Source-List-Change Records. 1227 These records indicate an explicit state change in a group at a 1228 system in either the group record's source list or its filter-mode. 1229 When a group membership is terminated at a system or traffic from a 1230 particular source is no longer desired, a multicast router must query 1231 for other members of the group or listeners of the source before 1232 deleting the group (or source) and pruning its traffic. 1234 To enable all systems on a network to respond to changes in group 1235 membership, multicast routers send specific queries. A Group- 1236 Specific Query is sent to verify there are no systems that desire 1237 reception of the specified group or to "rebuild" the desired 1238 reception state for a particular group. Group-Specific Queries are 1239 sent when a router receives a State-Change record indicating a system 1240 is leaving a group. 1242 A Group-and-Source Specific Query is used to verify there are no 1243 systems on a network which desire to receive traffic from a set of 1244 sources. Group-and-Source Specific Queries list sources for a 1245 particular group which have been requested to no longer be forwarded. 1246 This query is sent by a multicast router to learn if any systems 1247 desire reception of packets to the specified group address from the 1248 specified source addresses. Group-and-Source Specific Queries are 1249 only sent in response to State-Change Records and never in response 1250 to Current-State Records. Section 4.1.11 describes each query in 1251 more detail. 1253 6.2. IGMP State Maintained by Multicast Routers 1255 Multicast routers implementing IGMPv3 keep state per group per 1256 attached network. This group state consists of a filter-mode, a list 1257 of sources, and various timers. For each attached network running 1258 IGMP, a multicast router records the desired reception state for that 1259 network. That state conceptually consists of a set of records of the 1260 form: 1262 (multicast address, group timer, filter-mode, (source records)) 1264 Each source record is of the form: 1266 (source address, source timer) 1268 If all sources within a given group are desired, an empty source 1269 record list is kept with filter-mode set to EXCLUDE. This means 1270 hosts on this network want all sources for this group to be 1271 forwarded. This is the IGMPv3 equivalent to a IGMPv1 or IGMPv2 group 1272 join. 1274 6.2.1. Definition of Router Filter-Mode 1276 To reduce internal state, IGMPv3 routers keep a filter-mode per group 1277 per attached network. This filter-mode is used to condense the total 1278 desired reception state of a group to a minimum set such that all 1279 systems' memberships are satisfied. This filter-mode may change in 1280 response to the reception of particular types of group records or 1281 when certain timer conditions occur. In the following sections, we 1282 use the term "router filter-mode" to refer to the filter-mode of a 1283 particular group within a router. Section 6.4 describes the changes 1284 of a router filter-mode per group record received. 1286 Conceptually, when a group record is received, the router filter-mode 1287 for that group is updated to cover all the requested sources using 1288 the least amount of state. As a rule, once a group record with a 1289 filter-mode of EXCLUDE is received, the router filter-mode for that 1290 group will be EXCLUDE. 1292 When a router filter-mode for a group is EXCLUDE, the source record 1293 list contains two types of sources. The first type is the set which 1294 represents conflicts in the desired reception state; this set must be 1295 forwarded by some router on the network. The second type is the set 1296 of sources which hosts have requested to not be forwarded. 1297 Appendix A describes the reasons for keeping this second set when in 1298 EXCLUDE mode. 1300 When a router filter-mode for a group is INCLUDE, the source record 1301 list is the list of sources desired for the group. This is the total 1302 desired set of sources for that group. Each source in the source 1303 record list must be forwarded by some router on the network. 1305 Because a reported group record with a filter-mode of EXCLUDE will 1306 cause a router to transition its filter-mode for that group to 1307 EXCLUDE, a mechanism for transitioning a router's filter-mode back to 1308 INCLUDE must exist. If all systems with a group record in EXCLUDE 1309 filter-mode cease reporting, it is desirable for the router filter- 1310 mode for that group to transition back to INCLUDE mode. This 1311 transition occurs when the group timer expires and is explained in 1312 detail in Section 6.5. 1314 6.2.2. Definition of Group Timers 1316 The group timer is only used when a group is in EXCLUDE mode and it 1317 represents the time for the filter-mode of the group to expire and 1318 switch to INCLUDE mode. We define a group timer as a decrementing 1319 timer with a lower bound of zero kept per group per attached network. 1320 Group timers are updated according to the types of group records 1321 received. 1323 A group timer expiring when a router filter-mode for the group is 1324 EXCLUDE means there are no listeners on the attached network in 1325 EXCLUDE mode. At this point, a router will transition to INCLUDE 1326 filter-mode. Section 6.5 describes the actions taken when a group 1327 timer expires while in EXCLUDE mode. 1329 The following table summarizes the role of the group timer. 1330 Section Section 6.4 describes the details of setting the group timer 1331 per type of group record received. 1333 +-------------+-------+---------------------------------------------+ 1334 | Group | Group | Actions/Comments | 1335 | Filter-Mode | Timer | | 1336 | | Value | | 1337 +-------------+-------+---------------------------------------------+ 1338 | INCLUDE | Timer | All members in INCLUDE mode. | 1339 | | >= 0 | | 1340 | EXCLUDE | Timer | At least one member in EXCLUDE mode. | 1341 | | > 0 | | 1342 | EXCLUDE | Timer | No more listeners to group. If all source | 1343 | | == 0 | timers have expired then | 1344 | | | delete Group Record. If there are still | 1345 | | | source record timers running, | 1346 | | | switch to INCLUDE filter-mode using those | 1347 | | | source records with running | 1348 | | | timers as the INCLUDE source record state. | 1349 +-------------+-------+---------------------------------------------+ 1351 6.2.3. Definition of Source Timers 1353 A source timer is kept per source record and is a decrementing timer 1354 with a lower bound of zero. Source timers are updated according to 1355 the type and filter-mode of the group record received. Source timers 1356 are always updated (for a particular group) whenever the source is 1357 present in a received record for that group. Section 6.4 describes 1358 the setting of source timers per type of group records received. 1360 A source record with a running timer with a router filter-mode for 1361 the group of INCLUDE means that there is currently one or more 1362 systems (in INCLUDE filter-mode) which desire to receive that source. 1363 If a source timer expires with a router filter-mode for the group of 1364 INCLUDE, the router concludes that traffic from this particular 1365 source is no longer desired on the attached network, and deletes the 1366 associated source record. 1368 Source timers are treated differently when a router filter-mode for a 1369 group is EXCLUDE. If a source record has a running timer with a 1370 router filter-mode for the group of EXCLUDE, it means that at least 1371 one system desires the source. It should therefore be forwarded by a 1372 router on the network. Appendix A describes the reasons for keeping 1373 state for sources that have been requested to be forwarded while in 1374 EXCLUDE state. 1376 If a source timer expires with a router filter-mode for the group of 1377 EXCLUDE, the router informs the routing protocol that there is no 1378 longer a receiver on the network interested in traffic from this 1379 source. 1381 When a router filter-mode for a group is EXCLUDE, source records are 1382 only deleted when the group timer expires. Section 6.3 describes the 1383 actions that should be taken dependent upon the value of a source 1384 timer. 1386 6.3. IGMPv3 Source-Specific Forwarding Rules 1388 When a multicast router receives a datagram from a source destined to 1389 a particular group, a decision has to be made whether to forward the 1390 datagram onto an attached network or not. The multicast routing 1391 protocol in use is in charge of this decision, and should use the 1392 IGMPv3 information to ensure that all sources/groups desired on a 1393 subnetwork are forwarded to that subnetwork. IGMPv3 information does 1394 not override multicast routing information; for example, if the 1395 IGMPv3 filter-mode group for G is EXCLUDE, a router may still forward 1396 packets for excluded sources to a transit subnet. 1398 To summarize, the following table describes the forwarding 1399 suggestions made by IGMP to the routing protocol for traffic 1400 originating from a source destined to a group. It also summarizes 1401 the actions taken upon the expiration of a source timer based on the 1402 router filter-mode of the group. 1404 +-------------+----------+------------------------------------------+ 1405 | Group | Group | Action | 1406 | Filter-Mode | Timer | | 1407 | | Value | | 1408 +-------------+----------+------------------------------------------+ 1409 | INCLUDE | TIMER > | Suggest to forward traffic from source | 1410 | | 0 | | 1411 | INCLUDE | TIMER == | Suggest to stop forwarding traffic from | 1412 | | 0 | source and remove source record. If | 1413 | | | there are no more source | 1414 | | | records for the group, delete group | 1415 | | | record. | 1416 | INCLUDE | No | Suggest to not forward source | 1417 | | Source | | 1418 | | Elements | | 1419 | EXCLUDE | TIMER > | Suggest to forward traffic from source | 1420 | | 0 | | 1421 | EXCLUDE | TIMER == | Suggest to not forward traffic from | 1422 | | 0 | source (DO NOT remove record) | 1423 | EXCLUDE | No | Suggest to forward traffic from source | 1424 | | Source | | 1425 | | Elements | | 1426 +-------------+----------+------------------------------------------+ 1428 6.4. Action on Reception of Reports 1430 6.4.1. Reception of Current-State Records 1432 When receiving Current-State Records, a router updates both its group 1433 and source timers. In some circumstances, the reception of a type of 1434 group record will cause the router filter-mode for that group to 1435 change. The table below describes the actions, with respect to state 1436 and timers that occur to a router's state upon reception of Current- 1437 State Records. 1439 The following notation is used to describe the updating of source 1440 timers. The notation ( A, B ) will be used to represent the total 1441 number of sources for a particular group, where 1443 A = set of source records whose source timers > 0 (Sources that at 1444 least one host has requested to be forwarded) 1445 B = set of source records whose source timers = 0 (Sources that IGMP 1446 will suggest to the routing protocol not to forward) 1448 Note that there will only be two sets when a router's filter-mode for 1449 a group is EXCLUDE. When a router's filter-mode for a group is 1450 INCLUDE, a single set is used to describe the set of sources 1451 requested to be forwarded (e.g., simply (A)). 1453 In the following tables, abbreviations are used for several variables 1454 (all of which are described in detail in Section 8). The variable 1455 GMI is an abbreviation for the Group Membership Interval, which is 1456 the time in which group memberships will time out. The variable LMQT 1457 is an abbreviation for the Last Member Query Time, which is the total 1458 time spent after Last Member Query Count retransmissions. LMQT 1459 represents the "leave latency", or the difference between the 1460 transmission of a membership change and the change in the information 1461 given to the routing protocol. 1463 Within the "Actions" section of the router state tables, we use the 1464 notation 'A=J', which means that the set A of source records should 1465 have their source timers set to value J. 'Delete A' means that the 1466 set A of source records should be deleted. 'Group Timer=J' means 1467 that the Group Timer for the group should be set to value J. 1469 Router State Report Rec'd New Router State Actions 1470 ------------ ------------ ---------------- ------- 1472 INCLUDE (A) IS_IN (B) INCLUDE (A+B) (B)=GMI 1474 INCLUDE (A) IS_EX (B) EXCLUDE (A*B,B-A) (B-A)=0 1475 Delete (A-B) 1476 Group Timer=GMI 1478 EXCLUDE (X,Y) IS_IN (A) EXCLUDE (X+A,Y-A) (A)=GMI 1480 EXCLUDE (X,Y) IS_EX (A) EXCLUDE (A-Y,Y*A) (A-X-Y)=GMI 1481 Delete (X-A) 1482 Delete (Y-A) 1483 Group Timer=GMI 1485 6.4.2. Reception of Filter-Mode-Change and Source-List-Change Records 1487 When a change in the global state of a group occurs in a system, the 1488 system sends either a Source-List-Change Record or a Filter-Mode- 1489 Change Record for that group. As with Current-State Records, routers 1490 must act upon these records and possibly change their own state to 1491 reflect the new desired membership state of the network. 1493 Routers must query sources that are requested to be no longer 1494 forwarded to a group. When a router queries or receives a query for 1495 a specific set of sources, it lowers its source timers for those 1496 sources to a small interval of Last Member Query Time seconds. If 1497 group records are received in response to the queries which express 1498 interest in receiving traffic from the queried sources, the 1499 corresponding timers are updated. 1501 Similarly, when a router queries a specific group, it lowers its 1502 group timer for that group to a small interval of Last Member Query 1503 Time seconds. If any group records expressing EXCLUDE mode interest 1504 in the group are received within the interval, the group timer for 1505 the group is updated and the suggestion to the routing protocol to 1506 forward the group stands without any interruption. 1508 During a query period (i.e., Last Member Query Time seconds), the 1509 IGMP component in the router continues to suggest to the routing 1510 protocol that it forwards traffic from the groups or sources that it 1511 is querying. It is not until after Last Member Query Time seconds 1512 without receiving a record expressing interest in the queried group 1513 or sources that the router may prune the group or sources from the 1514 network. 1516 The following table describes the changes in group state and the 1517 action(s) taken when receiving either Filter-Mode-Change or Source- 1518 List-Change Records. This table also describes the queries which are 1519 sent by the querier when a particular report is received. 1521 We use the following notation for describing the queries which are 1522 sent. We use the notation 'Q(G)' to describe a Group-Specific Query 1523 to G. We use the notation 'Q(G,A)' to describe a Group-and-Source 1524 Specific Query to G with source-list A. If source-list A is null as 1525 a result of the action (e.g., A*B) then no query is sent as a result 1526 of the operation. 1528 In order to maintain protocol robustness, queries sent by actions in 1529 the table below need to be transmitted [Last Member Query Count] 1530 times, once every [Last Member Query Interval]. 1532 If while scheduling new queries, there are already pending queries to 1533 be retransmitted for the same group, the new and pending queries have 1534 to be merged. In addition, received host reports for a group with 1535 pending queries may affect the contents of those queries. 1536 Section Section 6.6.3 describes the process of building and 1537 maintaining the state of pending queries. 1539 Router State Report Rec'd New Router State Actions 1540 ------------ ------------ ---------------- ------- 1542 INCLUDE (A) ALLOW (B) INCLUDE (A+B) (B)=GMI 1544 INCLUDE (A) BLOCK (B) INCLUDE (A) Send Q(G,A*B) 1546 INCLUDE (A) TO_EX (B) EXCLUDE (A*B,B-A) (B-A)=0 1547 Delete (A-B) 1548 Send Q(G,A*B) 1549 Group Timer=GMI 1551 INCLUDE (A) TO_IN (B) INCLUDE (A+B) (B)=GMI 1552 Send Q(G,A-B) 1554 EXCLUDE (X,Y) ALLOW (A) EXCLUDE (X+A,Y-A) (A)=GMI 1556 EXCLUDE (X,Y) BLOCK (A) EXCLUDE (X+(A-Y),Y) (A-X-Y)=Group Timer 1557 Send Q(G,A-Y) 1559 EXCLUDE (X,Y) TO_EX (A) EXCLUDE (A-Y,Y*A) (A-X-Y)=Group Timer 1560 Delete (X-A) 1561 Delete (Y-A) 1562 Send Q(G,A-Y) 1563 Group Timer=GMI 1565 EXCLUDE (X,Y) TO_IN (A) EXCLUDE (X+A,Y-A) (A)=GMI 1566 Send Q(G,X-A) 1567 Send Q(G) 1569 6.5. Switching Router Filter-Modes 1571 The group timer is used as a mechanism for transitioning the router 1572 filter-mode from EXCLUDE to INCLUDE. 1574 When a group timer expires with a router filter-mode of EXCLUDE, a 1575 router assumes that there are no systems with a filter-mode of 1576 EXCLUDE present on the attached network. When a router's filter-mode 1577 for a group is EXCLUDE and the group timer expires, the router 1578 filter-mode for the group transitions to INCLUDE. 1580 A router uses source records with running source timers as its state 1581 for the switch to a filter-mode of INCLUDE. If there are any source 1582 records with source timers greater than zero (i.e., requested to be 1583 forwarded), a router switches to filter-mode of INCLUDE using those 1584 source records. Source records whose timers are zero (from the 1585 previous EXCLUDE mode) are deleted. 1587 For example, if a router's state for a group is EXCLUDE(X,Y) and the 1588 group timer expires for that group, the router switches to filter- 1589 mode of INCLUDE with state INCLUDE(X). 1591 6.6. Action on Reception of Queries 1593 6.6.1. Timer Updates 1595 When a router sends or receives a query with a clear Suppress Router- 1596 Side Processing flag, it must update its timers to reflect the 1597 correct timeout values for the group or sources being queried. The 1598 following table describes the timer actions when sending or receiving 1599 a Group-Specific or Group-and-Source Specific Query with the Suppress 1600 Router-Side Processing flag not set. 1602 +--------+---------------------------------------------------+ 1603 | Query | Action | 1604 +--------+---------------------------------------------------+ 1605 | Q(G,A) | Source Timer for sources in A are lowered to LMQT | 1606 | Q(G) | Group Timer is lowered to LMQT | 1607 +--------+---------------------------------------------------+ 1609 When a router sends or receives a query with the Suppress Router-Side 1610 Processing flag set, it will not update its timers. 1612 6.6.2. Querier Election 1614 IGMPv3 elects a single querier per subnet using the same querier 1615 election mechanism as IGMPv2, namely by IP address. When a router 1616 receives a query with a lower IP address, it sets the Other-Querier- 1617 Present timer to Other Querier Present Interval and ceases to send 1618 queries on the network if it was the previously elected querier. 1619 After its Other-Querier Present timer expires, it should begin 1620 sending General Queries. 1622 If a router receives an older version query, it MUST use the oldest 1623 version of IGMP on the network. For a detailed description of 1624 compatibility issues between IGMP versions see section Section 7. 1626 6.6.3. Building and Sending Specific Queries 1628 6.6.3.1. Building and Sending Group Specific Queries 1630 When a table action "Send Q(G)" is encountered, then the group timer 1631 must be lowered to LMQT. The router must then immediately send a 1632 group specific query as well as schedule [Last Member Query Count - 1633 1] query retransmissions to be sent every [Last Member Query 1634 Interval] over [Last Member Query Time]. 1636 When transmitting a group specific query, if the group timer is 1637 larger than LMQT, the "Suppress Router-Side Processing" bit is set in 1638 the query message. 1640 6.6.3.2. Building and Sending Group and Source Specific Queries 1642 When a table action "Send Q(G,X)" is encountered by a querier in the 1643 table in Section 6.4.2, the following actions must be performed for 1644 each of the sources in X of group G, with source timer larger than 1645 LMQT: 1647 o Set number of retransmissions for each source to [Last Member 1648 Query Count]. 1650 o Lower source timer to LMQT. 1652 The router must then immediately send a group and source specific 1653 query as well as schedule [Last Member Query Count - 1] query 1654 retransmissions to be sent every [Last Member Query Interval] over 1655 [Last Member Query Time]. The contents of these queries are 1656 calculated as follows. 1658 When building a group and source specific query for a group G, two 1659 separate query messages are sent for the group. The first one has 1660 the "Suppress Router-Side Processing" bit set and contains all the 1661 sources with retransmission state and timers greater than LMQT. The 1662 second has the "Suppress Router-Side Processing" bit clear and 1663 contains all the sources with retransmission state and timers lower 1664 or equal to LMQT. If either of the two calculated messages does not 1665 contain any sources, then its transmission is suppressed. 1667 Note: If a group specific query is scheduled to be transmitted at the 1668 same time as a group and source specific query for the same group, 1669 then transmission of the group and source specific message with the 1670 "Suppress Router-Side Processing" bit set may be suppressed. 1672 7. Interoperation With Older Versions of IGMP 1674 IGMP version 3 hosts and routers interoperate with hosts and routers 1675 that have not yet been upgraded to IGMPv3. This compatibility is 1676 maintained by hosts and routers taking appropriate actions depending 1677 on the versions of IGMP operating on hosts and routers within a 1678 network. 1680 7.1. Query Version Distinctions 1682 The IGMP version of a Membership Query message is determined as 1683 follows: 1685 IGMPv1 Query: length = 8 octets AND Max Resp Code field is zero 1687 IGMPv2 Query: length = 8 octets AND Max Resp Code field is non- 1688 zero 1690 IGMPv3 Query: length >= 12 octets 1692 Query messages that do not match any of the above conditions (e.g., a 1693 Query of length 10 octets) MUST be silently ignored. 1695 7.2. Group Member Behavior 1697 7.2.1. In the Presence of Older Version Queriers 1699 In order to be compatible with older version routers, IGMPv3 hosts 1700 MUST operate in version 1 and version 2 compatibility modes. IGMPv3 1701 hosts MUST keep state per local interface regarding the compatibility 1702 mode of each attached network. A host's compatibility mode is 1703 determined from the Host Compatibility Mode variable which can be in 1704 one of three states: IGMPv1, IGMPv2 or IGMPv3. This variable is kept 1705 per interface and is dependent on the version of General Queries 1706 heard on that interface as well as the Older Version Querier Present 1707 timers for the interface. 1709 In order to switch gracefully between versions of IGMP, hosts keep 1710 both an IGMPv1 Querier Present timer and an IGMPv2 Querier Present 1711 timer per interface. IGMPv1 Querier Present is set to Older Version 1712 Querier Present Timeout seconds whenever an IGMPv1 Membership Query 1713 is received. IGMPv2 Querier Present is set to Older Version Querier 1714 Present Timeout seconds whenever an IGMPv2 General Query is received. 1716 The Host Compatibility Mode of an interface changes whenever an older 1717 version query (than the current compatibility mode) is heard or when 1718 certain timer conditions occur. When the IGMPv1 Querier Present 1719 timer expires, a host switches to Host Compatibility mode of IGMPv2 1720 if it has a running IGMPv2 Querier Present timer. If it does not 1721 have a running IGMPv2 Querier Present timer then it switches to Host 1722 Compatibility of IGMPv3. When the IGMPv2 Querier Present timer 1723 expires, a host switches to Host Compatibility mode of IGMPv3. 1725 The Host Compatibility Mode variable is based on whether an older 1726 version General query was heard in the last Older Version Querier 1727 Present Timeout seconds. The Host Compatibility Mode is set 1728 depending on the following: 1730 +-----------------+-------------------------------------------------+ 1731 | Host | Timer State | 1732 | Compatibility | | 1733 | Mode | | 1734 +-----------------+-------------------------------------------------+ 1735 | IGMPv3 | IGMPv2 Querier Present not running and IGMPv1 | 1736 | (default) | Querier Present not running | 1737 | IGMPv2 | IGMPv2 Querier Present running and IGMPv1 | 1738 | | Querier Present not running | 1739 | IGMPv1 | IGMPv1 Querier Present running | 1740 +-----------------+-------------------------------------------------+ 1742 If a host receives a query which causes its Querier Present timers to 1743 be updated and correspondingly its compatibility mode, it should 1744 switch compatibility modes immediately. 1746 When Host Compatibility Mode is IGMPv3, a host acts using the IGMPv3 1747 protocol on that interface. When Host Compatibility Mode is IGMPv2, 1748 a host acts in IGMPv2 compatibility mode, using only the IGMPv2 1749 protocol, on that interface. When Host Compatibility Mode is IGMPv1, 1750 a host acts in IGMPv1 compatibility mode, using only the IGMPv1 1751 protocol on that interface. 1753 An IGMPv1 router will send General Queries with the Max Resp Code set 1754 to 0. This MUST be interpreted as a value of 100 (10 seconds). 1756 An IGMPv2 router will send General Queries with the Max Resp Code set 1757 to the desired Max Resp Time, i.e., the full range of this field is 1758 linear and the exponential algorithm described in Section 4.1.1 is 1759 not used. 1761 Whenever a host changes its compatibility mode, it cancels all its 1762 pending response and retransmission timers. 1764 7.2.2. In the Presence of Older Version Group Members 1766 An IGMPv3 host may be placed on a network where there are hosts that 1767 have not yet been upgraded to IGMPv3. A host MAY allow its IGMPv3 1768 Membership Record to be suppressed by either a Version 1 Membership 1769 Report, or a Version 2 Membership Report. 1771 7.3. Multicast Router Behavior 1773 7.3.1. In the Presence of Older Version Queriers 1775 IGMPv3 routers may be placed on a network where at least one router 1776 on the network has not yet been upgraded to IGMPv3. The following 1777 requirements apply: 1779 o If any older versions of IGMP are present on routers, the querier 1780 MUST use the lowest version of IGMP present on the network. This 1781 must be administratively assured; routers that desire to be 1782 compatible with IGMPv1 and IGMPv2 MUST have a configuration option 1783 to act in IGMPv1 or IGMPv2 compatibility modes. When in IGMPv1 1784 mode, routers MUST send Periodic Queries with a Max Resp Code of 0 1785 and truncated at the Group Address field (i.e., 8 bytes long), and 1786 MUST ignore Leave Group messages. They SHOULD also warn about 1787 receiving an IGMPv2 or IGMPv3 query, although such warnings MUST 1788 be rate-limited. When in IGMPv2 mode, routers MUST send Periodic 1789 Queries truncated at the Group Address field (i.e., 8 bytes long), 1790 and SHOULD also warn about receiving an IGMPv3 query (such 1791 warnings MUST be rate-limited). They also MUST fill in the Max 1792 Resp Time in the Max Resp Code field, i.e., the exponential 1793 algorithm described in Section 4.1.1 is not used. 1795 o If a router is not explicitly configured to use IGMPv1 or IGMPv2 1796 and hears an IGMPv1 Query or IGMPv2 General Query, it SHOULD log a 1797 warning. These warnings MUST be rate-limited. 1799 7.3.2. In the Presence of Older Version Group Members 1801 IGMPv3 routers may be placed on a network where there are hosts that 1802 have not yet been upgraded to IGMPv3. In order to be compatible with 1803 older version hosts, IGMPv3 routers MUST operate in version 1 and 1804 version 2 compatibility modes. IGMPv3 routers keep a compatibility 1805 mode per group record. A group's compatibility mode is determined 1806 from the Group Compatibility Mode variable which can be in one of 1807 three states: IGMPv1, IGMPv2 or IGMPv3. This variable is kept per 1808 group record and is dependent on the version of Membership Reports 1809 heard for that group as well as the Older Version Host Present timer 1810 for the group. 1812 In order to switch gracefully between versions of IGMP, routers keep 1813 an IGMPv1 Host Present timer and an IGMPv2 Host Present timer per 1814 group record. The IGMPv1 Host Present timer is set to Older Version 1815 Host Present Timeout seconds whenever an IGMPv1 Membership Report is 1816 received. The IGMPv2 Host Present timer is set to Older Version Host 1817 Present Timeout seconds whenever an IGMPv2 Membership Report is 1818 received. 1820 The Group Compatibility Mode of a group record changes whenever an 1821 older version report (than the current compatibility mode) is heard 1822 or when certain timer conditions occur. When the IGMPv1 Host Present 1823 timer expires, a router switches to Group Compatibility mode of 1824 IGMPv2 if it has a running IGMPv2 Host Present timer. If it does not 1825 have a running IGMPv2 Host Present timer then it switches to Group 1826 Compatibility of IGMPv3. When the IGMPv2 Host Present timer expires 1827 and the IGMPv1 Host Present timer is not running, a router switches 1828 to Group Compatibility mode of IGMPv3. Note that when a group 1829 switches back to IGMPv3 mode, it takes some time to regain source- 1830 specific state information. Source-specific information will be 1831 learned during the next General Query, but sources that should be 1832 blocked will not be blocked until [Group Membership Interval] after 1833 that. 1835 The Group Compatibility Mode variable is based on whether an older 1836 version report was heard in the last Older Version Host Present 1837 Timeout seconds. The Group Compatibility Mode is set depending on 1838 the following: 1840 +-------------------+-----------------------------------------------+ 1841 | Group | Timer State | 1842 | Compatibility | | 1843 | Mode | | 1844 +-------------------+-----------------------------------------------+ 1845 | IGMPv3 (default) | IGMPv2 Host Present not running and IGMPv1 | 1846 | | Host Present not running | 1847 | IGMPv2 | IGMPv2 Host Present running and IGMPv1 Host | 1848 | | Present not running | 1849 | IGMPv1 | IGMPv1 Host Present running | 1850 +-------------------+-----------------------------------------------+ 1852 If a router receives a report which causes its older Host Present 1853 timers to be updated and correspondingly its compatibility mode, it 1854 SHOULD switch compatibility modes immediately. 1856 When Group Compatibility Mode is IGMPv3, a router acts using the 1857 IGMPv3 protocol for that group. 1859 When Group Compatibility Mode is IGMPv2, a router internally 1860 translates the following IGMPv2 messages for that group to their 1861 IGMPv3 equivalents: 1863 +----------------+-------------------+ 1864 | IGMPv2 Message | IGMPv3 Equivalent | 1865 +----------------+-------------------+ 1866 | Report | IS_EX( {} ) | 1867 | Leave | TO_IN( {} ) | 1868 +----------------+-------------------+ 1870 IGMPv3 BLOCK messages are ignored, as are source-lists in TO_EX() 1871 messages (i.e., any TO_EX() message is treated as TO_EX( {} )). 1873 When Group Compatibility Mode is IGMPv1, a router internally 1874 translates the following IGMPv1 and IGMPv2 messages for that group to 1875 their IGMPv3 equivalents: 1877 +----------------+-------------------+ 1878 | IGMPv2 Message | IGMPv3 Equivalent | 1879 +----------------+-------------------+ 1880 | v1 Report | IS_EX( {} ) | 1881 | v2 Report | IS_EX( {} ) | 1882 +----------------+-------------------+ 1884 In addition to ignoring IGMPv3 BLOCK messages and source-lists in 1885 TO_EX() messages as in IGMPv2 Group Compatibility Mode, IGMPv2 Leave 1886 messages and IGMPv3 TO_IN() messages are also ignored. 1888 8. List of Timers, Counters and Their Default Values 1890 Most of these timers are configurable. If non-default settings are 1891 used, they MUST be consistent among all systems on a single link. 1892 Note that parentheses are used to group expressions to make the 1893 algebra clear. 1895 8.1. Robustness Variable 1897 The Robustness Variable allows tuning for the expected packet loss on 1898 a network. If a network is expected to be lossy, the Robustness 1899 Variable may be increased. IGMP is robust to (Robustness Variable - 1900 1) packet losses. The Robustness Variable MUST NOT be zero, and 1901 SHOULD NOT be one. Default: 2 1903 8.2. Query Interval 1905 The Query Interval is the interval between General Queries sent by 1906 the Querier. Default: 125 seconds. 1908 By varying the [Query Interval], an administrator may tune the number 1909 of IGMP messages on the network; larger values cause IGMP Queries to 1910 be sent less often. 1912 8.3. Query Response Interval 1914 The Max Response Time used to calculate the Max Resp Code inserted 1915 into the periodic General Queries. Default: 100 (10 seconds) 1917 By varying the [Query Response Interval], an administrator may tune 1918 the burstiness of IGMP messages on the network; larger values make 1919 the traffic less bursty, as host responses are spread out over a 1920 larger interval. The number of seconds represented by the [Query 1921 Response Interval] must be less than the [Query Interval]. 1923 8.4. Group Membership Interval 1925 The Group Membership Interval is the amount of time that must pass 1926 before a multicast router decides there are no more members of a 1927 group or a particular source on a network. 1929 This value MUST be ((the Robustness Variable) times (the Query 1930 Interval)) plus (one Query Response Interval). 1932 8.5. Other Querier Present Interval 1934 The Other Querier Present Interval is the length of time that must 1935 pass before a multicast router decides that there is no longer 1936 another multicast router which should be the querier. This value 1937 MUST be ((the Robustness Variable) times (the Query Interval)) plus 1938 (one half of one Query Response Interval). 1940 8.6. Startup Query Interval 1942 The Startup Query Interval is the interval between General Queries 1943 sent by a Querier on startup. Default: 1/4 the Query Interval. 1945 8.7. Startup Query Count 1947 The Startup Query Count is the number of Queries sent out on startup, 1948 separated by the Startup Query Interval. Default: the Robustness 1949 Variable. 1951 8.8. Last Member Query Interval 1953 The Last Member Query Interval is the Max Response Time used to 1954 calculate the Max Resp Code inserted into Group-Specific Queries sent 1955 in response to Leave Group messages. It is also the Max Response 1956 Time used in calculating the Max Resp Code for Group-and-Source- 1957 Specific Query messages. Default: 10 (1 second) 1958 Note that for values of LMQI greater than 12.8 seconds, a limited set 1959 of values can be represented, corresponding to sequential values of 1960 Max Resp Code. When converting a configured time to a Max Resp Code 1961 value, it is recommended to use the exact value if possible, or the 1962 next lower value if the requested value is not exactly representable. 1964 This value may be tuned to modify the "leave latency" of the network. 1965 A reduced value results in reduced time to detect the loss of the 1966 last member of a group or source. 1968 8.9. Last Member Query Count 1970 The Last Member Query Count is the number of Group-Specific Queries 1971 sent before the router assumes there are no local members. The Last 1972 Member Query Count is also the number of Group-and-Source-Specific 1973 Queries sent before the router assumes there are no listeners for a 1974 particular source. Default: the Robustness Variable. 1976 8.10. Last Member Query Time 1978 The Last Member Query Time is the time value represented by the Last 1979 Member Query Interval, multiplied by the Last Member Query Count. It 1980 is not a tunable value, but may be tuned by changing its components. 1982 8.11. Unsolicited Report Interval 1984 The Unsolicited Report Interval is the time between repetitions of a 1985 host's initial report of membership in a group. Default: 1 second. 1987 8.12. Older Version Querier Present Interval 1989 The Older Version Querier Present Interval is the timeout for 1990 transitioning a host back to IGMPv3 mode once an older version query 1991 is heard. When an older version query is received, hosts set their 1992 Older Version Querier Present Timer to Older Version Querier Present 1993 Interval. 1995 It is RECOMMENDED to use the default values for calculating the 1996 interval value as hosts do not know the values configured on the 1997 querying routers. This value SHOULD be [Robustness Variable] times 1998 [Query Interval] plus (10 times the Max Resp Time in the last 1999 received query message). 2001 8.13. Older Host Present Interval 2003 The Older Host Present Interval is the time-out for transitioning a 2004 group back to IGMPv3 mode once an older version report is sent for 2005 that group. When an older version report is received, routers set 2006 their Older Host Present Timer to Older Host Present Interval. 2008 This value MUST be ((the Robustness Variable) times (the Query 2009 Interval)) plus (one Query Response Interval). 2011 8.14. Configuring Timers 2013 This section is meant to provide advice to network administrators on 2014 how to tune these settings to their network. Ambitious router 2015 implementations might tune these settings dynamically based upon 2016 changing characteristics of the network. 2018 8.14.1. Robustness Variable 2020 The Robustness Variable tunes IGMP to expected losses on a link. 2021 IGMPv3 is robust to (Robustness Variable - 1) packet losses, e.g., if 2022 the Robustness Variable is set to the default value of 2, IGMPv3 is 2023 robust to a single packet loss but may operate imperfectly if more 2024 losses occur. On lossy subnetworks, the Robustness Variable should 2025 be increased to allow for the expected level of packet loss. 2026 However, increasing the Robustness Variable increases the leave 2027 latency of the subnetwork. (The leave latency is the time between 2028 when the last member stops listening to a source or group and when 2029 the traffic stops flowing.) 2031 8.14.2. Query Interval 2033 The overall level of periodic IGMP traffic is inversely proportional 2034 to the Query Interval. A longer Query Interval results in a lower 2035 overall level of IGMP traffic. The Query Interval MUST be equal to 2036 or longer than the Max Response Time inserted in General Query 2037 messages. 2039 8.14.3. Max Response Time 2041 The burstiness of IGMP traffic is inversely proportional to the Max 2042 Response Time. A longer Max Response Time will spread Report 2043 messages over a longer interval. However, a longer Max Response Time 2044 in Group-Specific and Source-and-Group-Specific Queries extends the 2045 leave latency. (The leave latency is the time between when the last 2046 member stops listening to a source or group and when the traffic 2047 stops flowing.) The expected rate of Report messages can be 2048 calculated by dividing the expected number of Reporters by the Max 2049 Response Time. The Max Response Time may be dynamically calculated 2050 per Query by using the expected number of Reporters for that Query as 2051 follows: 2053 +---------------------------+---------------------------------------+ 2054 | Query Type | Expected number of Reporters | 2055 +---------------------------+---------------------------------------+ 2056 | General Query | All systems on subnetwork | 2057 | Group-Specific Query | All systems that had expressed | 2058 | | interest in the group on the | 2059 | | subnetwork | 2060 | Source-and-Group-Specific | All systems on the subnetwork that | 2061 | Query | had expressed interest in | 2062 | | the source and group | 2063 +---------------------------+---------------------------------------+ 2065 A router is not required to calculate these populations or tune the 2066 Max Response Time dynamically; these are simply guidelines. 2068 9. Security Considerations 2070 We consider the ramifications of a forged message of each type, and 2071 describe the usage of IPSEC AH to authenticate messages if desired. 2073 9.1. Query Message 2075 A forged Query message from a machine with a lower IP address than 2076 the current Querier will cause Querier duties to be assigned to the 2077 forger. If the forger then sends no more Query messages, other 2078 routers' Other Querier Present timer will time out and one will 2079 resume the role of Querier. During this time, if the forger ignores 2080 Leave Messages, traffic might flow to groups with no members for up 2081 to [Group Membership Interval]. 2083 A DoS attack on a host could be staged through forged Group-and- 2084 Source-Specific Queries. The attacker can find out about membership 2085 of a specific host with a general query. After that it could send a 2086 large number of Group-and-Source-Specific queries, each with a large 2087 source list and the Maximum Response Time set to a large value. The 2088 host will have to store and maintain the sources specified in all of 2089 those queries for as long as it takes to send the delayed response. 2090 This would consume both memory and CPU cycles in order to augment the 2091 recorded sources with the source lists included in the successive 2092 queries. 2094 To protect against such a DoS attack, a host stack implementation 2095 could restrict the number of Group-and-Source-Specific Queries per 2096 group membership within this interval, and/or record only a limited 2097 number of sources. 2099 Forged Query messages from the local network can be easily traced. 2100 There are three measures necessary to defend against externally 2101 forged Queries: 2103 Routers SHOULD NOT forward Queries. This is easier for a router 2104 to accomplish if the Query carries the Router-Alert option. 2106 Hosts SHOULD ignore v2 or v3 Queries without the Router-Alert 2107 option. 2109 Hosts SHOULD ignore v1, v2 or v3 General Queries sent to a 2110 multicast address other than 224.0.0.1, the all-systems address. 2112 9.2. Current-State Report messages 2114 A forged Report message may cause multicast routers to think there 2115 are members of a group on a network when there are not. Forged 2116 Report messages from the local network are meaningless, since joining 2117 a group on a host is generally an unprivileged operation, so a local 2118 user may trivially gain the same result without forging any messages. 2119 Forged Report messages from external sources are more troublesome; 2120 there are two defenses against externally forged Reports: 2122 Ignore the Report if you cannot identify the source address of the 2123 packet as belonging to a network assigned to the interface on 2124 which the packet was received. This solution means that Reports 2125 sent by mobile hosts without addresses on the local network will 2126 be ignored. Report messages with a source address of 0.0.0.0 2127 SHOULD be accepted on any interface. 2129 Ignore Report messages without Router Alert options [RFC2113], and 2130 require that routers not forward Report messages. (The 2131 requirement is not a requirement of generalized filtering in the 2132 forwarding path, since the packets already have Router Alert 2133 options in them.) This solution breaks backwards compatibility 2134 with implementations of IGMPv1 or earlier versions of IGMPv2 which 2135 did not require Router Alert. 2137 A forged Version 1 Report Message may put a router into "version 1 2138 members present" state for a particular group, meaning that the 2139 router will ignore Leave messages. This can cause traffic to flow to 2140 groups with no members for up to [Group Membership Interval]. This 2141 can be solved by providing routers with a configuration switch to 2142 ignore Version 1 messages completely. This breaks automatic 2143 compatibility with Version 1 hosts, so should only be used in 2144 situations where "fast leave" is critical. 2146 A forged Version 2 Report Message may put a router into "version 2 2147 members present" state for a particular group, meaning that the 2148 router will ignore IGMPv3 source-specific state messages. This can 2149 cause traffic to flow from unwanted sources for up to [Group 2150 Membership Interval]. This can be solved by providing routers with a 2151 configuration switch to ignore Version 2 messages completely. This 2152 breaks automatic compatibility with Version 2 hosts, so should only 2153 be used in situations where source include and exclude is critical. 2155 9.3. State-Change Report Messages 2157 A forged State-Change Report message will cause the Querier to send 2158 out Group-Specific or Source-and-Group-Specific Queries for the group 2159 in question. This causes extra processing on each router and on each 2160 member of the group, but can not cause loss of desired traffic. 2161 There are two defenses against externally forged State-Change Report 2162 messages: 2164 Ignore the State-Change Report message if you cannot identify the 2165 source address of the packet as belonging to a subnet assigned to 2166 the interface on which the packet was received. This solution 2167 means that State-Change Report messages sent by mobile hosts 2168 without addresses on the local subnet will be ignored. State- 2169 Change Report messages with a source address of 0.0.0.0 SHOULD be 2170 accepted on any interface. 2172 Ignore State-Change Report messages without Router Alert options 2173 [RFC2113], and require that routers not forward State-Change 2174 Report messages. (The requirement is not a requirement of 2175 generalized filtering in the forwarding path, since the packets 2176 already have Router Alert options in them.) 2178 9.4. 9.4. IPSEC Usage 2180 In addition to these measures, IPSEC in Authentication Header mode 2181 [RFC2402] may be used to protect against remote attacks by ensuring 2182 that IGMPv3 messages came from a system on the LAN (or, more 2183 specifically, a system with the proper key). When using IPSEC, the 2184 messages sent to 224.0.0.1 and 224.0.0.22 should be authenticated 2185 using AH. When keying, there are two possibilities: 2187 1. Use a symmetric signature algorithm with a single key for the LAN 2188 (or a key for each group). This allows validation that a packet 2189 was sent by a system with the key. This has the limitation that 2190 any system with the key can forge a message; it is not possible 2191 to authenticate the individual sender precisely. It also 2192 requires disabling IPSec's Replay Protection. 2194 2. When appropriate key management standards have been developed, 2195 use an asymmetric signature algorithm. All systems need to know 2196 the public key of all routers, and all routers need to know the 2197 public key of all systems. This requires a large amount of key 2198 management but has the advantage that senders can be 2199 authenticated individually so e.g., a host cannot forge a message 2200 that only routers should be allowed to send. 2202 This solution only directly applies to Query and Leave messages in 2203 IGMPv1 and IGMPv2, since Reports are sent to the group being reported 2204 and it is not feasible to agree on a key for host-to-router 2205 communication for arbitrary multicast groups. 2207 10. IANA Considerations 2209 All IGMP types described in this document are already assigned in 2210 [RFC3228]. 2212 11. Contributors 2214 Brad Cain, Steve Deering, Isidor Kouvelas, Bill Fenner, and Ajit 2215 Thyagarajan are the authors of RFC 3376, which forms the bulk of the 2216 content contained herein. 2218 12. Acknowledgments 2220 We would like to thank Ran Atkinson, Luis Costa, Toerless Eckert, 2221 Dino Farinacci, Serge Fdida, Wilbert de Graaf, Sumit Gupta, Mark 2222 Handley, Bob Quinn, Michael Speer, Dave Thaler and Rolland Vida for 2223 comments and suggestions on RFC 3376. 2225 13. References 2227 13.1. Normative References 2229 [RFC1112] Deering, S., "Host extensions for IP multicasting", STD 5, 2230 RFC 1112, DOI 10.17487/RFC1112, August 1989, 2231 . 2233 [RFC2113] Katz, D., "IP Router Alert Option", RFC 2113, 2234 DOI 10.17487/RFC2113, February 1997, 2235 . 2237 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 2238 Requirement Levels", BCP 14, RFC 2119, 2239 DOI 10.17487/RFC2119, March 1997, 2240 . 2242 [RFC2236] Fenner, W., "Internet Group Management Protocol, Version 2243 2", RFC 2236, DOI 10.17487/RFC2236, November 1997, 2244 . 2246 [RFC2402] Kent, S. and R. Atkinson, "IP Authentication Header", 2247 RFC 2402, DOI 10.17487/RFC2402, November 1998, 2248 . 2250 [RFC3228] Fenner, B., "IANA Considerations for IPv4 Internet Group 2251 Management Protocol (IGMP)", BCP 57, RFC 3228, 2252 DOI 10.17487/RFC3228, February 2002, 2253 . 2255 13.2. Informative References 2257 [RFC1071] Braden, R., Borman, D., and C. Partridge, "Computing the 2258 Internet checksum", RFC 1071, DOI 10.17487/RFC1071, 2259 September 1988, . 2261 [RFC3376] Cain, B., Deering, S., Kouvelas, I., Fenner, B., and A. 2262 Thyagarajan, "Internet Group Management Protocol, Version 2263 3", RFC 3376, DOI 10.17487/RFC3376, October 2002, 2264 . 2266 [RFC3569] Bhattacharyya, S., Ed., "An Overview of Source-Specific 2267 Multicast (SSM)", RFC 3569, DOI 10.17487/RFC3569, July 2268 2003, . 2270 [RFC3678] Thaler, D., Fenner, B., and B. Quinn, "Socket Interface 2271 Extensions for Multicast Source Filters", RFC 3678, 2272 DOI 10.17487/RFC3678, January 2004, 2273 . 2275 Appendix A. Design Rationale 2277 A.1. The Need for State-Change Messages 2279 IGMPv3 specifies two types of Membership Reports: Current-State and 2280 State Change. This section describes the rationale for the need for 2281 both these types of Reports. 2283 Routers need to distinguish Membership Reports that were sent in 2284 response to Queries from those that were sent as a result of a change 2285 in interface state. Membership reports that are sent in response to 2286 Membership Queries are used mainly to refresh the existing state at 2287 the router; they typically do not cause transitions in state at the 2288 router. Membership Reports that are sent in response to changes in 2289 interface state require the router to take some action in response to 2290 the received report (see Section 6.4). 2292 The inability to distinguish between the two types of reports would 2293 force a router to treat all Membership Reports as potential changes 2294 in state and could result in increased processing at the router as 2295 well as an increase in IGMP traffic on the network. 2297 A.2. Host Suppression 2299 In IGMPv1 and IGMPv2, a host would cancel sending a pending 2300 membership reports if a similar report was observed from another 2301 member on the network. In IGMPv3, this suppression of host 2302 membership reports has been removed. The following points explain 2303 the reasons behind this decision. 2305 1. Routers may want to track per-host membership status on an 2306 interface. This allows routers to implement fast leaves (e.g., 2307 for layered multicast congestion control schemes) as well as 2308 track membership status for possible accounting purposes. 2310 2. Membership Report suppression does not work well on bridged LANs. 2311 Many bridges and Layer2/Layer3 switches that implement IGMP 2312 snooping do not forward IGMP messages across LAN segments in 2313 order to prevent membership report suppression. Removing 2314 membership report suppression eases the job of these IGMP 2315 snooping devices. 2317 3. By eliminating membership report suppression, hosts have fewer 2318 messages to process; this leads to a simpler state machine 2319 implementation. 2321 4. In IGMPv3, a single membership report now bundles multiple 2322 multicast group records to decrease the number of packets sent. 2323 In comparison, the previous versions of IGMP required that each 2324 multicast group be reported in a separate message. 2326 A.3. Switching Router Filter Modes from EXCLUDE to INCLUDE 2328 If there exist hosts in both EXCLUDE and INCLUDE modes for a single 2329 multicast group in a network, the router must be in EXCLUDE mode as 2330 well (see section 6.2.1). In EXCLUDE mode, a router forwards traffic 2331 from all sources unless that source exists in the exclusion source 2332 list. If all hosts in EXCLUDE mode cease to exist, it would be 2333 desirable for the router to switch back to INCLUDE mode seamlessly 2334 without interrupting the flow of traffic to existing receivers. 2336 One of the ways to accomplish this is for routers to keep track of 2337 all sources desired by hosts that are in INCLUDE mode even though the 2338 router itself is in EXCLUDE mode. If the group timer now expires in 2339 EXCLUDE mode, it implies that there are no hosts in EXCLUDE mode on 2340 the network (otherwise a membership report from that host would have 2341 refreshed the group timer). The router can then switch to INCLUDE 2342 mode seamlessly with the list of sources currently being forwarded in 2343 its source list. 2345 Appendix B. Summary of Changes from IGMPv2 2347 While the main additional feature of IGMPv3 is the addition of source 2348 filtering, the following is a summary of other changes from RFC 2236. 2350 o State is maintained as Group + List-of-Sources, not simply Group 2351 as in IGMPv2. 2353 o Interoperability with IGMPv1 and IGMPv2 systems is defined as 2354 operations on the IGMPv3 state. 2356 o The IP Service Interface has changed to allow specification of 2357 source-lists. 2359 o The Querier includes its Robustness Variable and Query Interval in 2360 Query packets to allow synchronization of these variables on non- 2361 Queriers. 2363 o The Max Response Time in Query messages has an exponential range, 2364 changing the maximum from 25.5 seconds to about 53 minutes, for 2365 use on links with huge numbers of systems. 2367 o Hosts retransmit state-change messages for increased robustness. 2369 o Additional data sections are defined to allow later extensions. 2371 o Report packets are sent to 224.0.0.22, to assist layer-2 switches 2372 in snooping. 2374 o Report packets can contain multiple group records, to allow 2375 reporting of full current state using fewer packets. 2377 o Hosts no longer perform suppression, to simplify implementations 2378 and permit explicit membership tracking. 2380 o New Suppress Router-Side Processing (S) flag in Query messages 2381 fixes robustness issues which were also present in IGMPv2. 2383 Appendix C. Summary of Changes from RFC 3376 2385 The following is a list of changes made since RFC 3376. 2387 o Modified definition of Older Version Querier Present Interval to 2388 address Erratum 4375. 2390 Author's Address 2392 Brian Haberman (editor) 2393 Johns Hopkins University Applied Physics Lab 2395 Email: brian@innovationslab.net